In recent years, small, light-weighted and highly-dense electronic-products have been developed, and along with this demand for various types of printed-circuit board, particularly for a flexible printed-circuit board (may be referred to as “FPC” hereinafter), has been increasing. The FPC includes an insulative film and a circuit of metal foil formed thereon.
The flexible laminate such as the FPC is constituted of a flexible metal clad laminate. A typical flexible metal clad laminate is formed of an insulative material, and includes a flexible insulative film as a base substrate and a metal foil which is thermally pressed onto the surface of the base substrate via an adhesive material to be bonded thereto. A polyimide film is suitably used for the insulative film.
An epoxy or acrylic thermosetting adhesive is typically used as the adhesive material.
The flexible wire board using the thermosetting adhesive is made up of three layers: a base substrate, an adhesive material, and a metal foil, and is therefore also called a “three-layer FPC”.
The thermosetting adhesive used for the three-layer FPC has an advantage such that it ensures an adhesive property under a relatively low temperature. However, the three-layer FPC using the thermosetting adhesive is assumed to soon have a difficulty in meeting the increasing demand for a FPC with various characteristics such as heat-resistant, flexibility, or electric reliability at the same time.
In consideration of this demand, a recent technology has suggested a FPC in which a metal layer is directly formed on an insulative film, or includes thermoplastic polyimide as an adhesion layer. Because of the direct provision of metal layer on the insulative substrate, this FPC is called a two-layer FPC. The two-layer FPC has a superior characteristic than the three-layer FPC, and its property is sufficient to meet the requirement for the various characteristics. The demand for the two-layer FPC is therefore expected to increase in the future.
The flexible metal clad laminate used for a two-layer FPC is produced, for example, by (1) a casting method which casts or applies a polyamic acid, which is a precursor of polyimide, onto a metal foil, and imidizing the polyamic acid, (2) a metallization method which directly forms a metal layer on a polyimide film by sputtering or plating, or (3) a laminate method which bonds a polyimide film with a metal foil via thermoplastic polyimide.
Among these technologies, the laminate method can deal with a wider thickness range of metal foils than the casting method. Also, the device cost for the laminate method is lower than the metallization method. A typical apparatus used for the laminate method is a thermal roll laminate apparatus which carries out sequential laminate processes by unrolling a material roll, or a double belting press apparatus. Among them, the thermal roll laminate method using a thermal roll laminate apparatus is preferable in terms of productivity.
In recent small and light-weighted electronic devices, wires on the base substrate have become miniaturized, and the components mounted to the devices are also reduced in dimension to allow themselves to be disposed at a high density. Therefore, a significant change in dimension after the wire formation causes shifting of the components from the designed mounting position. This may cause inadequate conduction between the components and the base substrate.
The conventional laminate method for fabricating a three-layer FPC uses a thermosetting resin as the adhesion layer, and therefore the laminate temperature is less than 200° C. (see Patent Document 1). In contrast, the two-layer FPC includes thermoplastic polyimide as the adhesion layer, and therefore needs to be subjected to a heating process of 200° C. or greater, or even almost 400° C. in some cases. Therefore, residual strain remains on the flexible metal clad laminate resulted from the laminate process, which results in a dimensional change at the time of forming wires by etching, or at the time of reflow soldering for mounting components.
Particularly, the laminate method often has a process of casting or applying a polyamic acid, which is a precursor of thermoplastic polyimide, in forming an adhesion layer including thermoplastic polyimide on a polyimide film. In this case, since imidization is carried out by sequentially heating the casted or applied polyamic acid, and bonding of the metal foil is also carried out by sequential application of heat and pressure, the material is often is exposed to tension and heat. Therefore, a thermal stress in the MD direction and that in the TD direction differ. More specifically, a pulling force is applied in the MD direction under tension, and a contracting force is applied in the TD direction. As a result, in the removal of the metal foil from the flexible laminate by etching, and in the heating through the reflow soldering, the deformation is released. This causes a contraction in the MD direction, and an expansion in the TD direction.
Particularly, in recent years, a required solder resistance under the moisture absorption condition tends to be higher due to the usage of lead-free solders, and along with this the Tg (glass transition temperature) of the adhesion layer is also increasing. This results in an increase of the required temperature for the laminate process. Therefore, the thermal stress becomes further greater, more easily causing a dimensional change. This raises a necessity of material design which can ease the thermal stress.
In view of this problem, several techniques have been disclosed. They suggest suppression of the dimensional change by way of control of laminate pressure or tension control of the adhesive film (see Patent Document 2 or 3).
To be more specific, the Patent Document 2 discloses formation of a FPC which includes the steps of forming a metal foil and a heat-resistant adhesive film by heat and pressure by placing a protection material between at least one pair of press rolls; cooling the resulting lamination and the protection material which are in contact with light pressure; and separating the protection material from the lamination. This method has a smaller rate of dimensional change.
According to Patent Document 3, an adhesive film is dried in advance in the FPC production process to remove internal moisture, and the resulting adhesive film is subjected to thermal laminate as such. This manner prevents generation of irregular patterns on the surface of the lamination.
However, in the techniques of Patent Document 2 and Patent Document 3 which solve the dimensional change by changing the thermal laminate conditions, assurance of dimensional accuracy is limited. Therefore, there has been a demand for a dimensional change prevention technology ensuring high dimensional accuracy.
Further, the thickness of insulative layer used for the existing two-layer FPC is typically 25 μm (1 mil), but there also is a new demand for so-called a “half-mil” having a 12-15 μm thick insulative layer, in terms of further reduction in mounting space of base substrate, or the problem of spring back.
Compared with a 1-mil product, a half-mil product has a thinner adhesive film, which is more susceptible to the thermal stress on the laminate process, and therefore has a greater difficulty in suppressing dimensional change.
Under such a circumstance, there is a demand for a new technology for effectively suppressing a dimensional change of a polyimide film due to thermal stress.    [Patent Document 1] Japanese Unexamined Patent Publication “Tokukaihei 9-199830 (published on Jul. 31, 1997)”    [Patent Document 2] Japanese Unexamined Patent Publication “Tokukai 2002-326308 (published on Nov. 12, 2002)”    [Patent Document 3] Japanese Unexamined Patent Publication “Tokukai 2002-326280 (published on Nov. 12, 2002)”